Silicon carbide (that will be referred to as SiC) has a wide band gap, and its maximum dielectric breakdown electric field is larger than that of silicon by one order of magnitude. Thus, SiC has been highly expected to be used as a material for power semiconductor devices in the next generation. SiC is known as having crystalline polymorphism, namely, SiC crystallizes into two or more forms having different structures. In these days, single crystals, such as 6H-SiC and 4H-SiC, are being manufactured with sufficiently high quality. These crystals are alpha-phase SiC in which a zinc-blend structure and a wurtzite structure are superposed on each other.
Up to the present, various types of semiconductor devices, including Shottky diodes, vertical MOSFET, and thyristors, have been fabricated, and it has been confirmed that these devices exhibit better characteristics than conventional Si devices. In particular, 4H-SiC crystal is most expected to be applied to power devices, because the mobility of carriers in the 4H-SiC crystal has least dependence on its crystal orientation, and is higher than those of other forms of SiC.
Recently, MOS type semiconductor devices utilizing the MOS structure have been widely used as silicon semiconductor devices. In manufacturing the MOS type devices, a silicon substrate is exposed to an oxidizing atmosphere containing, for example, oxygen or water vapor, at a high temperature of 1000.degree. C. to 1200.degree. C., so as to form a silicon dioxide film (that will be referred to as "SiO.sub.2 film") on the surface of the substrate through the thermal oxidization. The SiO.sub.2 film thus formed is used as an insulating film.
In a similar method of thermal oxidization as used in the case of silicon, an SiO.sub.2 film can be grown on the surface of the SiC substrate, to provide an excellent semiconductor-insulator interface, as known in the art. Since the SiO.sub.2 film can be used as a gate insulating film or a stabilizing film, SiC may be similarly applied to MOS type semiconductor devices. This characteristic, i.e., formation of a favorable SiO.sub.2 film, is not generally observed in other compound semiconductors, and therefore can be advantageously utilized to easily manufacture MOS type semiconductor devices, such as MOSFET. Thus, SiC is expected to be used in a wide range of applications in the future.
A typical example of oxidizing process will be now explained. FIG. 5 is a graph representing a temperature program, which shows changes in the temperature during the typical oxidizing process. In FIG. 5, the horizontal axis indicates time, and the vertical axis indicates the temperature. Initially, a wafer is introduced into an oxidizing furnace at a temperature T.sub.2 that is lower than an oxidizing temperature, and the temperature of the furnace is then raised to the oxidizing temperature T.sub.3. Thereafter, the wafer is oxidized at the temperature T.sub.3 for a period of time t.sub.2. In this oxidizing process, an oxidizing atmosphere in the form of steam, or wet oxygen containing water vapor, or dry oxygen containing no water vapor, flows through the oxidizing furnace. After the oxidizing process, the wafer is subjected to an annealing process in suitable gas, at the same temperature as the oxidizing temperature, or other temperature, and then the furnace is cooled down. In the final step, the wafer is taken out of the furnace at a temperature T.sub.4. In the manufacturing process of silicon semiconductor devices, annealing in an inert gas, such as nitrogen or argon, is generally needed so as to lower the interface state density, for example. In FIG. 5, t.sub.3 represents a period of time for which annealing is conducted. Although the annealing temperature is equal to the oxidizing temperature in the above example, these temperatures may be different from each other.
As described above, an SiO.sub.2 film is grown on SiC by a similar method of thermal oxidization as employed for silicon. The SiO.sub.2 film and SiC provides a clean interface, and the SiO.sub.2 film grown on the SiC substrate can be used as a protective film or a gate insulating film of a MOS type semiconductor device.
Where the SiO.sub.2 film is formed on the SiC substrate through thermal oxidation, however, the interface state density between the SiO.sub.2 film and the SiC substrate is considerably higher than that of the silicon substrate, as reported in many publications (for example, Shenoy, J. N. et al.: J. of Electron Materials, Vol. 24, (1995) p.303). Such a high interface state density is fatal or detrimental to the MOS type semiconductor device that controls carried in a portion of the substrate that is very close to its surface. In this situation, several attempts have been made so as to lower the interface state density between the SiO.sub.2 film formed by thermal oxidization, and the SiC substrate.
For example, the inventors of the present invention found that the interface state density is significantly lowered, and the channel mobility of the MOSFET is improved, by employing a method in which the SiC wafer is exposed to an atmosphere consisting of water vapor during a cooling period after the thermal oxidation (as reported in Proceedings of International Conference on Silicon Carbide, III-nitrides and Related Materials-1997).
The characteristics of the interface may be improved by other methods in which the wafer is exposed to oxygen plasma or irradiated with ultraviolet rays as a preliminary treatment prior to oxidation (as reported by Zetterling C. M. et al.: Proceedings of the Sixth Internal Conference on Silicon Carbide and Related Materials 1995, Institute of Physics Conference Series Number 142, p.605).
Although the mobility of carriers in the 4H-SiC crystal has small dependence on the crystal orientation, and the 4H-SiC crystal provides a higher mobility than other crystal forms, currently available MOSFET using the 4H-SiC crystal does not have satisfactory characteristics. This is because the channel mobility of the MOSFET using the 4H-SiC crystal is considerably small, even though the mobility in a bulk of 4H-SiC crystal is relatively high. There have been no reliable report concerning the mobility or methods for improving the mobility.
The high interface state density arising when the thermal oxide film is formed on the surface of the SiC substrate is considered as one reason for the considerably small channel mobility. Thus, the channel mobility is expected to be improved by providing a desirable interface between the thermal oxide film and the SiC substrate.